The present invention relates generally to electrical capacitors which may be variably adjusted or tried, and more particularly to such capacitors which are electronically programmable, rather than mechanically set, to a desired value of capacitance. It is anticipated that a primary application of the present invention will be for adjusting or tuning more general electronic circuits employing the invention.
Many electronic circuits require adjustment of particular parameters, either during initial assembly or later, as changing conditions or deterioration due to age, etc. affect such parameters. Of present interest is capacitance, since setting capacitance is critically important to insure the proper operation of a wide variety of electrical circuits and circuit-based processes which are in wide use. Capacitance may, of course, be set by making an initial choice of or performing replacement with a fixed-value component. This will generally not be dealt with here, but it should be kept in mind that substituting an adjustable capacitor for a fixed-value one may be useful in many situations.
Capacitors which may be variably adjusted are desirable, or even critically necessary, in many electronic circuits. For example, computers, clocks, radios, televisions, garage door openers, and a myriad of other electronic devices all have one or more internal circuits that require correct capacitance value adjustment. Adjusting capacitance to achieve such values can be done at the time of initial assembly, where it is almost always necessary, or it can be done later by readjusting back to the original value or by changing to an entirely new capacitance value, as appropriate.
For purposes of the following discussion, the operation of adjusting a variable capacitance device is collectively termed xe2x80x9ctrimming,xe2x80x9d regardless of the specific device type. Further, the operation of setting a variable capacitance device within a more general circuit is collectively termed xe2x80x9ctuning.xe2x80x9d It should be appreciated that these definitions are broad and widely encompassing ones. For example, while some might consider tuning to be proprietary to operations in particular industry segments, such as the adjustment of radio frequency oscillators and amplifiers, and to therefore not also encompass operations like setting delay circuits or stabilizing digital memories, such restrictive interpretation is not intended and is not appropriate here.
Tuning requires determining what capacitance value will be correct in a circuit, and then providing and trimming a xe2x80x9ctuning capacitorxe2x80x9d in the circuit to that value. Unfortunately, this can be an expensive, time-consuming, and error prone process.
FIG. 1 (background art) is a diagram depicting a general circuit 1 requiring capacitive tuning. A conventional analog tuning device 2 is connected to the general circuit 1 for this purpose. The analog tuning device 2 may be as simple as a standard adjustable capacitor, or it may be a complex assembly used to achieve the net effect of adjusting capacitance.
FIG. 2 (background art) depicts the usual choice made today for the analog tuning device 2, a variable analog capacitor 3. Presently the most commonly used variable analog capacitors 3 are mechanical in nature. For example, one type includes several semicircular plates which are rotated relative to other semicircular plates which are fixed. The amount by which the respective plates overlap then determines the capacitance. If the rotating plates do not overlap the fixed plates at all, the capacitance is nominally zero, and if the plates completely overlap, the maximum capacitance of the device is reached. Adjusting to any capacitance between zero and the maximum is thus possible.
Unfortunately, this type of variable analog capacitor 3 has a number of disadvantages. For example, the mechanical relationships of the plates are much subject to undesirable change by shock and vibration. Other, more subtle, influences on the net capacitance are variation in temperature, pressure, and humidity. The dielectric, or inter-plate medium, in such capacitors is often air or another gas, and must usually be kept relatively contained and uncontaminated. Liquid or gel filled adjustable capacitors are also possible but are uncommon, due to concerns such as fill leakage past the seals around adjustment mechanism shafts, etc. Entirely solid dielectrics are not possible, since the plates must permit movement. In sum, variable analog capacitors 3 have numerous inherent characteristics that make them unreliable and failure prone.
Another consideration is utility. To adjust or to readjust the variable analog capacitor 3 requires physical access to perform the mechanical adjustment operation, which is typically rotation. In large and complex systems other components and entire other systems may obscure physical access. In small and compact systems such access may also be difficult, and can even subject surrounding components to potential damage. In hazardous locations, such as the ocean""s depths, physical access can be quite difficult, and in remote locations, such as those visited by space probes, physical access can be outright impossible.
Accordingly, what is needed is a new type of trimable capacitor for use in tuning circuits. This new type of capacitor should not employ mechanical trimming, preferably being electronically trimable. It should also be physically durable, employing all solid and robust materials. It is further desirable that this new type of capacitor permit easy capacitance value adjustment and readjustment.
Accordingly, it is an object of the present invention to provide an electronically trimable variable capacitance device.
Another object of the invention is to provide such a device which is programmable using digital electronic techniques.
Another object of the invention is to provide such a device having a memory that can be temporarily or permanently store information about a present capacitance value of the device, for later recall and use to set the device back to that capacitance value.
Another object of the invention is to provide a variable capacitance device that provides accurate and stable values of capacitance across a wide range of potentially present signal frequencies.
And another object of the invention is to provide a digitally programmable variable capacitance device that is not dependant on mechanical principles of operation, and thus one that avoids the problems which such principles introduce.
Briefly, one preferred embodiment of the present invention is an electronic trim capacitor having a first and second capacitance terminals across which a net capacitance is exhibited. A number of branch circuits are present, each including a capacitor that is electrically connected in series with at least one switch. An interface is provided for selectively programming the operation of each of the switches to electrically connect or disconnect its respective branch circuit to the first and second capacitance terminals. In this manner, the capacitors in each branch circuit are selectively included in or excluded from contributing to the net capacitance.
An advantage of the present invention is that it provides an electronically, rather than mechanically, programmable variable capacitance device that is easily digitally programmed to temporarily or permanently store a capacitance value even after it has been incorporated into another circuit.
Another advantage of the present invention is that it allows trimming in set capacitance value increments.
Another advantage of the present invention is that it allows trimming to a desired net capacitance value without necessarily using feedback, such as measuring capacitance to confirm attainment of the desired value.
Another advantage of the present invention is that it is highly accurate, stable and reliable, due its non-mechanical principles of operation and its ability to be incorporated into integrated circuits and sealed or encapsulated module type packages.
And another advantage of the present invention is that it is highly economical, both to manufacture and to use. It may be manufactured using relatively inexpensive materials and techniques, and it may similarly be used in more general electronic circuits by also employing relatively inexpensive techniques. And it may be trimmed, and thus in turn tune such more general circuits, using essentially heretofore unavailable techniques which are easy, fast, precise, and reliable.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.